I'm playing as a dunmer rogue and have noticed that whether or not a shopkeeper will talk seems pretty random.

I accepted a quest, then went back to talk, pressed 'goodbye' without actually saying one word and now when I try to interact again, he won't talk, to my kind.

Reloaded the save just before interacting with him the first time and now the work is already done and he won't talk at all.

What gives? How does reputation with NPCs work in this game?

I already found out that the quests are random as is whether or not he found someone else (I think). So, is getting the cold shoulder random as well then; and it doesn't mean I've actually lost any rep with him?

Yes, NPC responses are random. Having a better rep with various NPCs, a higher Personality, etc just gives you a better chance of getting a positive response. Thankfully, NPCs are plentiful, as are their quests.

Most NPCs will not interact with you in any way while their quest is active; this is normal.

And you get those "Oh, the work resolved itself/I hired the fighters guild/etc" sometimes. They are there you make the world seem more alive, as if there are other people out there doing work. I like them.

Alright, I see, that clarified it

Those NPCs can sometimes act like pricks despite how much you helped them.

"Hello Christopher Yemingcroft, any n--"

"I don't speak to your kind. Go away."

"But I got rid twice of the harpies in your property, Castle Necromaghan, saved your five cousins from the Dark Brotherhood and two daughters from the Thieves Guild!"

"You get no response."

"Right. I'll just sell you my loot then."

"You've bargained with me until the sun sets, friend. I'll accept this for 640 gold pieces."

"Friend!?"

"You get no response."

" :huh: "

Yeah, they sure are schizo (or bipolar rather) =P

Ha ha ha ha, these are funny indeed!